In the industrial field or the medical field, an image fiber is used. The image fiber typically has such a configuration that a plurality of cores are disposed within a cladding. The number of pixels (the number of the cores) of the image fiber is, for example, 1000 or greater. In many image fibers, 2000 to tens of thousands of the cores are disposed within a cladding. For the sake of convenience in use, the image fiber generally has an outer diameter of several mm or less. Therefore, a diameter of the core should be within the micro meter range.